Optical storage medium and process

ABSTRACT

A data storage process which includes an erasable high speed, high density, storage medium having a transition metal oxide layer where said oxide layer is capable of undergoing an optically readable chemical change when simultaneously exposed to heat and to light of a selected wavelength. An optically readable image is formed in selected regions of the oxide layer under ambient conditions which include O 2  by simultaneously exposing said layer to heat and to radiation in the blue-green or shorter wavelength spectrum. The image is erased by heating the entire medium using a furnace or by heating selected portions with IR radiation.

This invention was made with government support under Grant No.F30602-92-C0042 from the United States Air Force, Rome Laboratories. Thegovernment has certain rights in this invention.

This application is a division of application Ser. No. 08/481,818 filedJun. 7, 1995 which application is now U.S. Pat. No. 5,691,091.

BACKGROUND OF THE INVENTION

The present invention relates in general to a recording medium and morespecifically to an erasable, high speed, high density, optical storagemedium which is both erasable and writable upon demand.

Currently, data can be stored on a wide variety of materials usingvarious manipulable physical and/or chemical properties as stateindicators. For example, conventional magnetic storage media usingfloppy disks and hard disks utilize the sense of magnetization impressedon a selected region of a surface of a metal oxide such as Cr₂ O₃. Themagnetization can be changed on demand using an external field whichforms the basis for both writing and erasing data. The data is read bysimply measuring the existing sense of magnetization. Magnetic media,however, are relatively slow in all functions, less dense in informationstorage capacity than any optically based storage medium and vulnerableto a variety of electromagnetic phenomenon.

Currently available optical storage media typically involves the use ofmultiple layers of metal and plastic. These media are irreversiblymodified when the data is written on them by irradiating a memoryelement with activating radiation such as laser light. These media aretypically purchased with the data, such as music, being contained on thearticle. Other optical memory devices such as WORM (write once read manytimes) allow the user to impress the data, but again the writing processis irreversible.

There are two commercially available forms of erasable optical mediawhich are available in the prior art. One uses a polarization sensitiveread mechanism in conjunction with a thermomagnetic induced surfacepolarization change between the written and erased states. The other isbased on modulating the reflection/scattering optical properties of themedium using a light induced transition between amorphous andcrystalline states. The material requirements for the magnetoopticmedium are stringent, leading to low yield manufacturing. Furthermore,the basic magnetooptic read mechanism involves a very weak effect so thesupport in electronics must be elaborate, and the light must be veryhighly polarized. In both of these types of media currently on themarket, the time required to write data is at best on the order of 1μsec. Neither of these types of existing erasable optical media hassufficient durability or is sufficiently inexpensive. In view of theseshortcomings, there is a need for basic improvements to existingerasable optical memory systems of the type described above.

It can therefore be seen that there is a continuous need in the fieldfor a system which can provide the desirable attributes ofconventionally available optical memory systems, such as CD's, but willalso provide the advantage of being erasable. A further need is for asystem which would not require metal components, and thus provide forimmunity from damage by electromagnetic radiation.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome theproblems of the prior art described above, and to provide an opticalinformation storage system which utilizes light and its own intrinsicphotochromic properties to chemically encode information. The systemfurther provides the advantage of selectively erasing the encodedinformation by exposing the storage medium to a selected wavelength orheating in bulk and erasing the entire medium. Optionally, the entireimage can be erased by heating the entire medium using an oven.

The present invention is based upon the discovery that certain solidtransition metal oxides can be formed in layers on a supportingsubstrate and used to chemically encode information through the use oflight and their intrinsic photochromic properties. The solid metaloxides suitable for this invention are those which undergo photoinducedand thermoenhanced loss of gas phase O₂ to produce mixed valence oxidesand include WO₃, V₂ O₅, TiO₂ and MoO₃. Any other solid transition metaloxide which exhibits these characteristics is included within the scopeof this invention. In an embodiment where tungsten oxide is used, WO₃ isthe erased state, and both oxidation states W^(V) and W^(VI) are presentin the written state. The two oxidation states must have similar oridentical chemical sites. A particular oxide can be operationallyestablished for any possible choice of oxide by exposing a possiblecandidate oxide to blue-green or shorter wavelength light under vacuumand observing whether a color change occurs. Further, as discussed byDuffy ("Bonding Energy Levels & Bands in Inorganic Solids", J. A.Duffy,; 1990, Longman Scientific & Technical, Essex, UK, Copublished inthe US with John Wiley & Sons, Inc. New York, 1990) only those mixedvalence compounds having semiconductor or metallic electrical propertieswill work in this invention. Metal oxides known to form "tungstenbronze-like" materials as defined by Duffy, i.e. WO₃, MoO₃, and V₂ O₅are preferred embodiments of the present invention.

In the context of tungsten, Duffy (pp. 182-191) described appropriatemixed valence compounds using the formula

    M.sub.x W.sub.x.sup.v W.sub.1-X.sup.VI O.sub.3 (0<x<1)

where M may be either an alkali metal ion (with nominal +1 charge) or aproton. More generally, (see P. G. Dickens and M. S. Whittingham Quart.Rev. 22, 30(1968)) it has been stated that other metal ions are alsousable. There is currently some uncertainty in the primary scientificliterature (e.g. C. Bechinger, G. Oefinger, S. Herminghaus and P.Leiderer, J. Appl. Phys. 74, pp. 4527-4533(1993) on the need for a metalion or proton to produce the highly colored (blue in the case of WO₃starting material) state from the simple metal oxide. Therefore, x=0 issuitable for this invention, but use of M in the form of a dopant oradditive could have application in some embodiments of the presentinvention. Where a dopant or additive is used, M=H⁺, Li⁺, Na⁺, or K⁺.However, previously dried WO₃, MoO₃, or V₂ O₅, which is then allowed toequilibrate with typical ambient air is a suitable material for anembodiment of the invention. The above references to Duffy, Dickens etal. and Bechinger et al. are incorporated herein by reference.

The possible use of M in some embodiments of this invention stems fromthree types of considerations. First, the spectral properties of themedium can be varied by choice of M and this allows some tailoring ofthe medium to the spectral characteristics of available laser systems,i.e. colors. This means that reading and writing functions can beengineered to some extent based on the availability of laser systems.Second, the time and laser power required for switching between theerased and written states will depend to some extent on the nature of M.Third, the stability and overall durability of the written and erasedstate depends on the nature of M. These considerations are notindependent of each other and so a balancing will be required forengineering specific systems. In the discussion which follows, use of Mwill not be referred to further because the use of WO₃ and MoO₃ withoutany added M already affords a nearly perfect match between the laserchemical properties of the medium and various properties of YAG and GaAsbased laser systems. It should be understood that the scope of thisinvention includes the potential use of M in some embodiments.

Imaging is accomplished in combination with simultaneously exposing theoxide layer, under ambient conditions, including the presence of gasphase O₂, to a selected range of wavelengths of light in the blue-greenor shorter (254 nm to 575 nm) spectrum, and to infrared radiation (800nm to 10.6 μm) or heat, resulting in the formation of a chemicallyencoded readable image on the layer of said metal oxide. This image ispermanent in nature and can be stored indefinitely. As will be describedbelow in greater detail, the function of the infrared radiation is toheat the medium with spatial selectivity and thereby facilitate thewrite process in the exposed area. The image can be selectively erasedby simply exposing the oxide layer to infrared radiation (heat) in thepresence of gaseous O₂ which restores the exposed area to its originalstate prior to imaging. Optionally, the entire image can be erased byheating the entire medium using an oven. Oxides of the metals W and Mohave been found to be particularly suitable for use in the presentinvention.

In one embodiment of the present invention, a layer of WO₃ powder(particle size 1-10 μm) approximately 1 mm thick is formed on a 2 cmdiameter disk of fused quartz. In use, an optically readable image isformed on the WO₃ layer by simultaneously exposing the layer toblue-green light and infrared light in selected regions of the layer toform a permanent optically readable image thereon. The image, orportions of the image may be conveniently erased by exposing theselected portions of the image to infrared radiation in the presence ofgaseous O₂. The entire image can be erased by heating the entire mediumusing an oven. In addition to providing a convenient imaging systemwhich is erasable, the medium has all of the favorable advantages, highstorage density, high speed, and durability of conventional CD's.

An example of the theory or mechanism which is involved in the imagingsystem of the present invention can be illustrated for the transitionmetal oxide WO₃. The reaction which occurs during the imaging step inwhich the WO₃ layer is exposed to blue-green light and IR light is achange in color of the layer from bright yellow to dark blue with thereaction being described by equation 1 ##STR1## In its blue color form,the oxide is illustrated in equation 1 as W₂ O₅(s)

Because the WO₃ is the thermodynamic ground state for thetungsten-oxygen system, the reverse action is easily thermally drivenand is illustrated by equation 2 ##STR2## As will be hereinafterillustrated, the blue color oxide state will be shown as WO₂.5.

The change from yellow to blue and the reverse reaction from blue toyellow constitutes the write and erase modes of the medium,respectively. The transition can be easily observed or read usingabsorption, reflectance, or Raman scattering measurements in either theUV--visible or IR spectral regions by conventional techniques availablein the art. The Raman scattering utilized is not surface enhanced. Inreading a WO₃ based medium, the Raman scattering could be resonanceenhanced, but not surface enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational sectional view of one embodiment of adevice of the present invention.

FIG. 2 depicts a schematic diagram of apparatus for use in the reading,writing, and erasing according to the present invention.

FIG. 3 is a schematic diagram which depicts an enlarged view of therecording medium mounting area of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described in more detail with reference to theaccompanying drawings where in FIG. 1 an erasable optical storage mediumof the present invention is illustrated in the form partial sidesectional view of a disk 20 which comprises a supporting substrate 22containing a thin coherent layer or film of optical memory media 24 onthe substrate.

Substrate 22 may comprise any suitable material which is capable ofsupporting layer or film 24, and which is substantially inert andimpervious to the radiation and/or heat used in the imaging and eraseprocess of the present invention. The substrate must also haveappropriate thermal conductivity such that the infrared radiated spotcan achieve an appropriate temperature. Suitable substrates includefused silica, quartz, silicon, sapphire, plastics, and the like. Theoptical storage medium 20 is normally used in the form of a circulardisk. It should be understood, however, that other configurations mayalso be used depending on the application of the medium. For aparticular embodiment which is designed to be erased in bulk, thesubstrate and layers must be able to withstand repeated heating andcooling cycles.

The optical memory media 24 may comprise any compound or material whichundergoes a chemical change as in equations 1 and 2 when exposed toradiation of selected wavelengths under ambient conditions which includeO₂, and which change can be optically read or scanned. A furtherrequirement is that the encoded change or written data may beselectively erased upon exposure to infrared radiation or to heat in thepresence of O₂.

Preferred materials suitable for use as the imaging medium for layer 24comprise WO₃ and MoO₃.

The thickness of layer or film 24 should be at least on the order ofabout 10² nm. A suitable range for the thickness of layer 24 would befrom about 0.1-10 μm. The layer 24 may be in the form of a simplecoherent powder layer, a powder containing a small amount of adhesive tobind the layer to substrate 22, or it may be coated in a slurry ontosubstrate 12 in any suitable inert binder material. In addition to theabove, layer 24 can be formed on substrate 22 by other conventionaltechniques known to the art such as sputtering, vapor deposition orelectrochemical methods.

A disk suitable for use in the present invention was prepared asfollows: WO₃ powder available from Aldrich Corporation, (purity 99.995%) was evenly spread and compacted into a circular recess formed in themiddle of a 2 cm diameter fused quartz disk to form a coherent layer ofpowdered WO₃. The disk has a thickness of about 0.3 cm. The recess wasapproximately 0.2 cm in diameter and 0.1 cm deep. This disk or medium isused in demonstrating the present invention and is illustrated byreference character 7 in FIGS. 2 and 3 of the drawings.

FIG. 2 depicts a schematic diagram of the Raman apparatus employed todemonstrate the write, erase and read processes of the present inventionusing the disk prepared above. Although the process is described interms of laser pulses, it should be understood that for someapplications continuous wave (CW) lasers can be used. It should also beclear that no small metal coated spheres or other structures areemployed to achieve a surface enhanced Raman scattering effect as hasbeen used in other prior art processes.

We define a "pulse train" as a continuous series of light pulses each ofa given time duration. A mode locked, Q-switched Quantronix YAG laser 1is used to produce a pulse train of ≈100 psec duration pulses of 1.064μm light each separated by ≈12 nsec. This pulse train enters anelectro-optic pulse selector 2 and an 8 consecutive pulse section of thepulse train is selected each millisecond. Each 8 pulse series isdirected into a harmonic generator 3 which converts ≈10% of each 1.064μm pulse into a roughly equal time duration 0.532 μm wavelength pulse.Thus, for each infrared pulse which enters the (KDP) harmonic generator,two output pulses are generated. One is nearly identical to the input inevery respect except power content, but the other is of only half thewavelength, i.e. green 0.532 μm wavelength. The results obtainedcorrespond to an average power of ≈3-5 mW for the 0.532 μm wavelengthtrain and ≈50 mW for the 1.064 μm wavelength train.

Each of the pulse trains exits the harmonic generator and is directedindependently by mirrors 4 along separate paths. In FIG. 2, corner cubes5 are used to form two "delay lines" which allow for the distances eachcolor pulse train travels to be varied independently. This feature isnecessary to ensure that each pulse train arrives at the medium at thesame time. The two pulse trains are focussed by a single lens 6 onto thedisk or medium 7 to a single ≈300 μm diameter spot containing the layerof WO₃ (see also FIG. 3). Temporal and spatial overlap of the two pulsetrains were checked using fast photodiodes, an EG&G boxcar averager, anIR viewer, and a CCD camera, respectively.

The two pulse trains contact the medium (sample) surface after passingthrough a flat elliptical shaped 3 cm×5 cm, mirror 8 with a 4 mm hole 9in it as shown in FIG. 3. The mirror is placed ≈3-5 cm above the medium7, with the plane of the mirror oriented ≈45° with respect to thesurface of the medium. Some of the light (≈1/8) which is scattered orreflected from the medium strikes mirror 8 which contains hole 9 throughthe mirror thickness. A Raman signal is light which has a wavelengthshifted from an incident wavelength during the scattered process.

All the light which strikes mirror 8 is collected by an uncoatedaspheric lens 10. The light collected using this lens is directed off anuncoated turning mirror 11, through a holographic edge filter 12 (POC,Tuscon Ark.) and focussed with a final collection lens 13 to a ≈2 mmspot on the entrance spectrograph exit slit of an Instruments SA 0.32meter spectrograph (1200 grooves/mm). The spectrograph exit slit isfitted with the 1028 channel photodiode array of an EG&G OMA III system.The OMA III is operated in the gated mode such that it is triggered bythe Q-switch synchronization pulse with a fixed 1 nsec gate width. Theholographic edge filter is necessary to remove all the infrared and mostof the incident 0.532 μm light. The spectrograph/OMA III system 14allows quantification of the amount of light which is shifted inwavelength from the incident 0.532 μm light. In the present embodiment,this Raman signal is the read mechanism for the medium. In order toexpose the medium to light source(s), one need only block theappropriate light path(s) with an opaque object. In this way the mediumcan be exposed to either just 0.532 μm light for reading, both 0.532 μmand 1.064 μm for writing, or just 1.064 μm for erasing. To easilyobserve the reading, writing, or erasing processes, 2 minute exposuresto the appropriate pulse train(s) are adequate.

FIG. 3 depicts an enlarged view of the recording medium mounting area ofFIG. 2 at a higher scale illustrating the fused silica disk 7 containingthe WO₃ powder layer.

FIG. 3 depicts a schematic close-up view of the interaction zone, i.e.,the region where the two light sources overlap a single bit of digitaldata. Although, the process parameters for utilizing the recordingmedium of the present invention have been clearly established, thecurrent understanding of the microscopic mechanism involved in theprocesses associated with the invention may be somewhat incomplete. Thepertinent characteristics of WO₃ have been described to establishcontext. The interactions involving the light sources and the surface ofthe medium corresponding to: 1) writing; 2) erasing; and 3) reading datawere then considered. For completeness, and as an illustration of anunderstanding of the invention, the following theory and/or mechanismsare believed relevant to an understanding of the present invention.

If the medium is initially in the erased form, i.e., nominally WO₃, itis yellow and possesses strong Raman features at 716 cm⁻¹ and 805 cm⁻¹.Unless otherwise stated, in what follows it can be assumed the medium isin contact with ambient air,. i.e., ≈20°-25° C. with total pressureroughly 760 torr. It is reasonable to expect that there is some waterand CO₂ present. While it is believed that the incident light or heatinduces some chemistry involving these species, these species do notpresently seem to have a significant part in the invention. The latticeof standard yellow WO₃ is formed from (WO₆) octahedral units. Theseoctahedral units are somewhat distorted and internal vibrations of theseoctahedral units, associated with the changing the type of distortion,are in fact the motions involved in producing the Raman features.

Assuming the medium is initially in the erased form, exposing it to apulse of infrared radiation (e.g. ≈1.06 μm) imparts heat to the mediumin proportion to the amount of light actually absorbed. However, in thepresent case, only a small amount of heat is absorbed because theabsorption/reflectivity spectrum of WO₃ suggests little absorption oflight near wavelengths of ≈1.06 μm. Whatever is absorbed initiallyexcites various motions in the medium. By means of the coupling betweenthe motions of electrons and nuclei, this absorption ultimately resultsin random motion of the nuclei and electrons comprising the lattice ofthe medium, i.e. heating. This simple heating mostly results inevaporation of some water, CO₂, or other impurities which is of noconsequence.

1. To describe the microscopic processes associated with writing dataonto the medium, it is presumed that a pulse of each color arrivessimultaneously to the erased medium surface. At first, the green lighthas a greater effect on the medium because it is absorbed to a greaterextent than the infrared. The green light initially excites electronsfrom valence bands which have at least some of their origin inwavefunctions associated with oxygen atoms. After electronic excitation,some electrons may be trapped by, or in the vicinity of, the W^(VI)metal ions at the centers of the octahedral units thereby forming theW^(V) metal ions responsible for the strong color change. In addition tothe well known color change, the presence of these ions in blue WO₂.5was confirmed by ESCA (electron spectroscopy for chemical analysis).Each W^(V) ion affects the bonding in the octahedral units in such a wayas to weaken the bonding between the metal and at least one oxygen atom.An observable effect of this bonding modification is the elimination ofthe two largest Raman features associated with WO₃. If an oxygen atom,whose bonding to its adjacent W^(VI) center is weakened, is in closeenough proximity to interact with another oxygen atom, it is possiblethat the two oxygen atoms will bond to each other forming an O₂molecule. If this O₂ molecule is sufficiently close to the surface ofthe medium then it may diffuse to, or on that surface and then desorbfrom that surface leaving behind a blue, oxygen deficient medium. Thisblue medium contains so called crystallographic shearplanes having bothlong and short range order. The yellow medium is composed of octahedrajoined at the corners. The blue material contains octahedra joined alongedges. Neither of these materials is amorphous. This is the basicmechanism of the write process, but there is another important part ofthe process which is essential to the present invention.

The absorption spectrum of WO₂.5 is very different from that of the WO₃in that is has a strong absorption starting in the red (≈700 nm),peaking around 1.06±0.25 μm and extending into the infrared. For thisreason, each time a W^(VI) is converted to W^(V), infrared radiation ismore strongly absorbed. The absorption of infrared heats and thereby"softens" the lattice, increasing the mobility of oxygen atoms andmolecules. The heating also helps to break various chemical bondsincluding metal-oxygen bonds. The cooperative effect of both colors isdifferent from the sum of the effects of each color applied separately.The greater the conversion of W^(VI) to W^(V), the greater theabsorption of infrared further enhancing the conversion. Whereas it ispossible to induce the conversion of W^(VI) to W^(V) using a singleblue-green or shorter wavelength color, at typical ambient atmosphericpressure and temperature much higher laser power is needed. Thisinvention allows lower power lasers to be used making the apparatusneeded to execute the process much easier to engineer. It may also allowincreased storage density by manipulation of the spatial and temporaloverlap between the two beams.

2. Having written a blue spot on the surface of the medium by convertingWO₃ to WO₂.5, as described above, the erase process can now beconsidered. This involves exposing the medium containing written data toinfrared radiation alone. The medium absorbs the infrared strongly whichagain results in heating and softening of the lattice. Collisionsbetween the gas phase O₂ and the various surface species are constantlyoccurring during the infrared exposure. The lattice heating facilitatesthe chemical reaction in which O₂ from the ambient air reacts with thesurface WO₂.5 to reform yellow WO₃. In the erase mode the blue-greenlight is not present so the reaction in which the W^(VI) is converted toW^(V) does not occur appreciably. The net effect of the infrared byitself interacting with the previously written medium is the conversionof blue W^(V) to yellow W^(VI) until the absorption of the infraredfalls sufficiently to limit the amount of heating which occurs. Asstated above this eventually happens because the yellow material has arelatively small absorption coefficient in the infrared (1.064 μm).

3. The read process can be accomplished using conventional reflectance,absorption or Raman scattering. It should be understood that any processwhich is selectively sensitive to the presence of WO₂.5 and WO₃ is apotential read mechanism. In the case of films this also includeselectrical properties. In each case only a small amount of one colorlight is incident on the medium surface which is not sufficient toeither heat the medium or, by electronic excitation, weaken many metalto oxygen bonds so no detectable chemical reactions occur. The data isread by detecting the amount of reflected or scattered light from themedium surface or by the amount of light which passes through a layer.These are standard conventional measurements known to anyone familiarwith modern spectroscopic techniques. The spectroscopic properties ofthe parent oxides and their derived mixed valence oxides are sodifferent that virtually any detection (read) scheme is many times moresensitive than the most favorable scheme currently being applied toexisting erasable optical memory.

Parameters Required to Effect the Data Storage Processes

In order to accomplish the writing process for the present invention, atleast two colors, i.e. wavelengths, of light are needed. One lightsource heats the medium. The other light source provides electronicexcitation to the medium. The "read" process may involve the electronicexcitation light source. The "write" process involves the simultaneoususe of both sources. The "erase" process involves only the heatingsource. In a further embodiment, it is also possible to utilize threesources. A "read" color, a "heat" color for erasing, and a combinationof the "heat" and a third color for writing. The heat source can be anyconventional infrared laser or furnace. Suitable lasers include YAG(wavelength=1.06 μm) or GaAs based lasers (≈850 nm).

The electronic excitation/read may be accomplished at any wavelength ofroughly 575 nm or shorter. This typically includes conventional ionlasers and dye lasers but most probably the sources of choices will beeither frequency doubled YAG(532 nm) or frequency doubled GaAs baseddevices (≈425 nm).

If use of a third color is desired, it will be so that reading can beaccomplished near an extremum of the reflectance curve of the writtenmedium. This will minimize read times and relax medium smoothnessrequirements on the read process. For the tungsten and molybdenum oxidebased media the optimal read color will be red. The lasers used in thepresent invention may be either pulsed or continuous wave (CW). EitherCW GaAs lasers, operated at less than 100% duty cycle, or YAG baseddevices would be suitable.

It may also be possible to use a single YAG laser as the source of allthe wavelengths even in the three color schemes. It may also be possibleto use an optical parametric oscillator. A single GaAs based lasersource can also be used to supply all the colors. For either a singleYAG or a GaAs device to provide all the colors, harmonic generationwould be used. This option has been commercially available in the fieldfor some time. If operated CW, any of the laser sources for heatingwould be adequate provided at least 10¹ -10² mW average power wereavailable.

In summary, the medium of the present invention has two statescorresponding to the written and erased states, respectively. The erasedstate corresponds to a fully oxygenated metal oxide as exemplified byyellow WO₃. The written state corresponds to a partially deoxygenatedmixed valence derivative of the same oxide as exemplified by blue WO₂.5.It is possible to attain the written state by simultaneously exposingthe yellow erased state to pulses of IR and blue-green or shorterwavelengths in ambient air or by exposing it to 514 or 488 nm continuouswave (CW) laser light (≈25 mW) under reduced oxygen partial pressure.The erased state can be reached from the written state by exposing themedium to a sufficiently intense IR radiation at atmospheric oxygenpressure or by simply heating it in ambient air in a furnace andallowing the medium to equilibrate. Using WO₃ as an example, thetransition in the furnace requires about 10¹ -10² seconds in a tubefurnace at about 400° C.

While specific embodiments of the invention have been illustrated anddescribed herein, it is realized that modifications and changes willoccur to those skilled in the art. It is therefore to be understood thatthe appended claims are intended to cover all modifications and changesas fall within the true spirit and scope of the invention.

What is claimed is:
 1. A data storage process which comprises:a.providing an erasable high speed, high density, storage medium whichincludes a transition metal oxide layer which is doped with a metal ionor proton in order to control the spectral properties of said oxidelayer and where said oxide layer is capable of undergoing an opticallyreadable chemical change when simultaneously exposed to heat and tolight of a selected wavelength; and b. simultaneously exposing saidlayer to heat and to radiation in the blue-green or shorter wavelengthspectrum under ambient conditions which include oxygen, whereby aphotochemical reaction is induced which includes an exchange of oxygen,resulting in the formation of an optically readable image in the exposedregions of said oxide layer.
 2. The process of claim 1 in which themetal oxide is selected from the group consisting of WO₃, MoO₃ and V₂O₅.
 3. A device which contains an optically readable image whichcomprises:a. an erasable high speed, high density, storage medium whichincludes a transition metal oxide layer where said oxide layer iscapable of undergoing an optically readable chemical change whensimultaneously exposed to heat and to light of a selected wavelength;and b. where said layer has been simultaneously exposed to heat and toradiation in the blue-green or shorter wavelength spectrum under ambientconditions which include oxygen whereby a photochemical reaction isinduced which includes an exchange of oxygen resulting in the formationof an optically readable image in the exposed regions of said oxidelayer.
 4. The device of claim 3 in which the metal oxide is an oxide ofany one of W. Mo. Ti. or V.
 5. The device of claim 3 where the metaloxide is doped with a metal ion or proton.
 6. The device of claim 3which further includes selectively erasing at least a portion of theimage formed in (b) above by exposing said image to infrared radiationin the presence of O₂.
 7. A device which contains an optically readableimage which comprises:(a) an erasable high speed, high density, storagemedium which includes a layer of WO₃, where said layer is capable ofundergoing an optically readable chemical change when simultaneouslyexposed to heat and to light of a selected wavelength; and (b) wheresaid layer has been simultaneously exposed to blue-green light orshorter wavelength spectrum, and infrared light in selective regions ofsaid layer, under ambient conditions including O₂ whereby aphoto-chemical reaction is induced which includes an exchange of oxygen,resulting in the formation of a permanent optically readable imagethereon.